MEMS (Microelectromechanical Systems) technology enables to fabricate microdevices with reduced size, weight, cost and power demanding, and yet with improved performance, functionality and reliability. The conventional or silicon-based MEMS fabrication techniques, which are derived directly from microelectronics or IC technologies, are not only planar (2-D) processes with low-aspect-ratios, but also material-limited. Rapid growth of MEMS calls for new fabrication techniques for manufacturing high-aspect-ratio complex three-dimensional (3-D) microdevices.
Multi-layer fabrication techniques are so far the most promising and versatile methods for fabricating complex three-dimensional microstructures with high-aspect-ratios for MEMS. U.S. Pat. No. 5,190,637, issued on Mar. 2, 1993 to Henry Guckel, and U.S. Pat. No. 6,027,630, issued on Feb. 22, 2000 to Adam L. Cohen, teach two fabrication methods for forming three-dimensional metal microstructures by using multi-layer electrochemical deposition. The only difference between the two methods is the way to perform the selective electrochemical deposition of metals. The essential elements of these two methods are described as follows.
(1) Three-dimensional microstructures are fabricated by using a successive layer-by-layer approach, meaning that a given layer can only be built after the pervious layer is completed.
(2) Each layer contains at least two different materials. In the case of a two-material system, one is a sacrificial material and the other is a structural material.
(3) Both structural material and sacrificial material are fabricated by using electrochemical deposition methods such as electrodeposition.
(4) After the deposition of both materials for building a given layer, a planarization process is used to machine the deposited materials to a predetermined thickness and to a flat and smooth surface as a base for building a next layer on it.
(5) After all layers have been built, the sacrificial material is etched to release the three-dimensional microstructures.
Although the above two multilayer electrochemical fabrication methods are practical for building complex three-dimensional microstructures, there are at least five(5) major drawbacks inherently associated with them, which are:
(1) Low throughput. Production throughput becomes an issue as the number of layers in the microdevices increases. This is because the above two multilayer fabrication methods are a successive layer-by-layer process. A given layer can only be built after the pervious layer is completed. The two main risks associated with a successive layer-by-layer process are: 1) When one layer meets fabrication problems, the fabrication process flow for this build has to be suspended until the problems are solved; and 2) If a build has to be discarded due to various reasons during fabrication, all previously fabricated layers have to be wasted.
(2) Low yield. In reality, each layer inevitably contains some defects. These defects may distribute at different locations on each layer. Therefore, the final yield of acceptable structures is an issue.
(3) Limited material selection. Electrochemical deposition can only be used to fabricate metals and alloys. It cannot or cannot easily make many important engineering and semiconductor materials such as stainless steel, aluminum and its alloys, titanium and its alloys, silicon, and non-metal materials. Further, although electrodeposition can be used to make many metals and alloys, only a small portion of them can be made practically and economically for mass production. Even among the metals and alloys that can be readily electrodeposited, there are those whose material properties are not as desired as those of the corresponding bulk materials. Therefore the multi-layer electrochemical fabrication methods taught in the two patents have limited applications.
(4) Layer thickness limitation. The maximum metal layer thickness is restricted by the mask thickness (e.g., photoresist). The ability to select the layer thickness freely is crucial to the multilayer approach. For example, in some cases, several successive thin layers may have to be deposited to reach a required thickness if a single thick layer cannot be formed. Obviously, this would increase production time, decrease production throughput and lower product yield.
(5) Structure geometry limitation. The multi-layer electrochemical fabrication methods can manufacture complex three-dimensional metal microstructures. However, there exist some types of shapes which cannot be fabricated. This structure geometry limitation is related to the etching of sacrificial material. If sacrificial material cannot be removed (e.g., in a sealed cavity with no etching access paths) or cannot be removed easily (e.g., in a long, narrow channel), microstructures with these shapes cannot be made with the two multi-layer methods.
U.S. Pat. No. 6,332,568 to Todd R. Christenson issued on Dec. 25, 2001 teaches a method to make multi-layer microstructures via diffusion bonding of subassemblies to form a united structure. This method allows the parallel manufacture of a series of subassemblies which compose a microstructure or micromachine. Each subassembly is built on a separate substrate. Then according to a predetermined configuration of the microstructure, the second subassembly is diffusion bonded onto the first subassembly. The substrate of the second subassembly is separated from the bonded two subassemblies after diffusion bonding. Therefore the substrate of a subassembly can be viewed as a sacrificial material as the substrate needs to be removed after one bonding operation is done. The above two steps (diffusion bonding and separation) are repeated until all the subassemblies are bonded to form a united structure. Compared with the multi-layer electrochemical fabrication approaches, the advantage of Christenson's approach is that all the required subassemblies for a microstructure can be built separately. However, the bonding of the subassemblies is still a successive process, meaning that the previous subassembly must be bonded before a given subassembly can be bonded. Diffusion bonding involves heat and pressure. Assume that a microstructure comprises ten subassemblies. Only the last (10th) subassembly undergoes heat and pressure treatment one time. All other nine subassemblies undergo heat and pressure treatment at least two times. The first two (1st and 2nd) subassemblies even undergo heat and pressure treatment nine times. The heat and pressure treatment may change the material properties of a microstructure or may distort the components of a microstructure. Therefore, practically this bonding method can only work for the very limited number of subassemblies although in theory there is no limit on the number of subassemblies that can be joined.
In addition, as each of two subassemblies to be bonded contains a substrate which increases the difficulty for the alignment of micro-components on the two substrates before bonding. The reason that Christenson's approach has to use a substrate as part of a subassembly is that the substrate works as a mechanical support for micro-components on it. Without this substrate, all components on the substrate will fall apart. It is very difficult to align two subassembly with their substrates as micro-components are located between two substrates during alignment. The micro-components on the two substrates could be damaged due to this difficult alignment as the components are not protected and cannot be protected. After two subassemblies are bonded together, an extra process step has to be operated to separate one of the substrates, which not only increases process time and cost, but also may damage the bonded micro-components.
Direct bonding of pre-micromachined partial microstructures such as silicon structures to form relatively complex three-dimensional microstructures is a common method in MEMS manufacturing. For examples, Luc G. Fréchette et al. reported a method for building a silicon micromachined air turbine by direct bonding of five bulk micromachined silicon wafers in their paper “High-Speed Microfabricated Silicon Turbomachinery and Fluid Film Bearings” published in Journal of Microelectromechanical Systems (Vol. 14, No. 1, pp. 141-152, 2005). However, this approach (bulk micromachining plus silicon bonding) has its intrinsic drawbacks that only makes it suitable for building devices with limited 3-D geometries. The drawbacks are summarized as follows.
When we fabricate a microdevice, we actually fabricate multiple same microdevices (i.e., dies) on a substrate, so-called batch manufacturing. If we divide a complex three-dimensional device or a group of three-dimensional devices into multiple layers, we will find that at least some of the layers comprise isolated or discrete features at die level, at wafer level (no mechanical connection between devices), or at both die level and wafer level. Note that a discrete feature means that it does not mechanically connect with other features on the same layer. Obviously this kind of layer cannot be made separately as discrete features will fall down. This is a fundamental drawback in bulk micromachining plus silicon bonding. To overcome this technical barrier, special fabrication approaches have to be implemented. For example, for making the layers containing discrete features, one solution is to use temporary silicon connections which support discrete silicon features. After all layers are bonded together, the temporary silicon connections are then cut to release the discrete features. However, this post-treatment only works for special designs. In addition, it is not a reliable and desirable approach as the removal of the mechanical connections is the last step of the process. If it fails, all previous work will be wasted. To avoid making the layers containing discrete features, one solution is to do multiple etching on both sides of silicon layers. However, deep etching can be performed only once on each side of a layer, which restricts to form more complex structures. Besides, this solution involves many process steps and faces difficult operations, low throughput and low yield.
Another intrinsic drawback of this technology (bulk micromachining plus silicon bonding) is related to deep etching processes such as DRIE (Deep Reactive Ion Etching). The DRIE etch rate distributes non-uniformly both locally and globally over a silicon wafer depending on feature geometry and feature distribution. Although the etch parameters of DRIE can be adjusted to an extent to lessen this effect, non-uniformity cannot be avoided. In addition, after DRIE, one has to accept as-is for both etched depth non-uniformity and etched surface smoothness since there does not exist a post-treatment process for improving non-uniformity and smoothness.
Summarily, although bulk micromachining plus silicon bonding is an approach to form relatively complex silicon microstructures, the intrinsic drawbacks of this approach restrict it to build limited 3-D microstructures and make it a quite complicated and low yield process as well. Microdevices have to be designed to fit into the capabilities of this approach so that desired geometries or structures have to be compromised or even sacrificed, and optimal performance usually cannot be achieved.